Reinforcing fillers are usually added to a matrix material to form high-strength composites. In order for the resulting composites to be useful, the reinforcing fillers must have a high load-bearing ability and binding affinity for the matrix. Carbon nanotubes (CNTs) have been added to matrix materials to form high-strength composites. However, the use of CNTs as reinforcing fillers, including multi-walled CNTs, has several disadvantages. Multi-walled CNTs have a tendency to pull out of, or slip from the matrix material, resulting in reduced load bearing ability. This is attributed to the fact that the interface between the matrix material and nanotube layers is very weak, thereby causing a “sword-in-sheath” type failure mechanism. Typically, only the outermost layer of multi-wall CNTs contributes to load bearing strength. (See, for example, D. Qian, et al. Appl. Phys. Lett., 76, 2868 (2000) and C. Bower, et al. Appl. Phys. Lett., 74, 3317 (1999)). Because of the weak van der Waals interaction between the CNTs cylindrical graphene sheets, improved bonding between carbon nanomaterials such as relatively “inert” CNTs and the matrix material is, therefore, essential for improved mechanical performance of the composite.
For high-strength CNT reinforced composites, the matrix material has to bind to the CNT reinforcing filler strongly (to prevent the two surfaces from slipping), so that an applied load (such as a tensile stress) can be transferred to the nanotubes. (See, for example, P. Calvert, Nature, 339, 210 (1999)). Several methods, including chemical functionalization of CNT tubule ends and side walls have been proposed and attempted to enhance bonding between CNTs and matrix material. (See, for example, J. Chen, et al. Science, 282, 95 (1998); A. Grag, et al. Chem. Phys. Lett., 295, 273 (1998), and S. Delpeux, et al. AIP Conf. Proc., 486, 470 (1999)). However, no significant improvement in mechanical properties has been observed after such modification. Chemical coating of both multi-wall and single-wall CNTs with metals and metallic oxides have also been reported for applications such as heterogeneous catalysis and one-dimensional nanoscale composites. (See, for example, T. W. Ebbesen, et al. Adv. Mater., 8, 155 (1996), X. Chen, et al. Compos. Sci. Technol., 60, 301 (2000), and L. M. Ang et al. Carbon, 38, 363 (2000)). The bonding between the coating materials and CNTs is, however, not strong enough to result in efficient load transfer. Thus, there exists a need in the art to improve the interaction between CNT reinforcing fillers and matrix materials in order to confer high mechanical strength to CNT reinforced composites and enable their commercial use in the manufacture of high-strength, light-weight mechanical and electrical device components.